Update RUBIES every time chruby is run

While helping someone get setup with chruby and ruby-install, it wasn't immediately obvious to them that they would have to source their profile or start a new terminal session to see a newly installed ruby. I think that either chruby should detect changes in the directories where Rubies are stored, or ruby-install could update the array after install. The former seems more flexible for different installation methods and manual changes/removal.